Piezoelectric resonators are well known in the art as electronic elements used to select and stabilize a particular frequency. Piezoelectric resonators are widely used in various kinds of civilian electronic equipment including mobile communication systems such as cellular, cordless, and satellite telephones, and military electronic equipment such as radars and weapons seekers. Piezoelectric resonators can also be used as sensors of temperature, pressure, force, acceleration, and mass. In all cases, the purpose of the resonator is to precisely determine the allowed frequency and to provide immunity against unwanted external environmental influences. The basic design of piezoelectric resonators is well known to those skilled in the art. The properties of the crystal resonator depend on the angles of cut as the piezoelectric crystal material is formed into a substrate, often in the form of a wafer. Metal electrodes are disposed upon the crystal wafer, which is mounted in a structure designed to hold the crystal wafer and to provide electrical contact to an external circuit. This crystal and holder assembly is called a piezoelectric resonator. Surface acoustic wave (SAW) resonators and shallow-bulk-acoustic-wave devices can be made to operate at frequencies from several MHz to well above 1 GHz.
The frequency of a piezoelectric resonator is affected by environmental stresses that deform the resonator, including vibration, shock, gravitational stress, temperature, aging, thermal hysteresis and so on. While these effects may be used as the basis for sensors, they are considered adverse effects in the case of a resonator. For example, even acceleration due to gravity produces measurable effects and the frequency of a piezoelectric resonator can shift significantly when turned upside down. In a specific example, when an oscillator using an ST-cut SAW resonator is turned upside down, the frequency typically shifts about 4×10−9 due to the acceleration sensitivity of approximately 2×10−9 g−1. The sensitivity is the same when the crystal is subjected to vibration, i.e., the time-varying acceleration due to the vibration modulates the resonator frequency at the vibration frequency with a peak deviation of ±2×10−9 g−1. In the frequency domain, the vibration sensitivity manifests itself as vibration-induced sidebands that appear at plus and minus the vibration frequency away from the carrier frequency.
The stresses caused by acceleration, vibration and shock are well-known to those skilled in the art. FIG. 1 illustrates a prior art piezoelectric surface wave resonator that suffers from inadequate structural rigidity and is therefore subject to excessive sensitivity arising from flexural deformations. The prior art piezoelectric surface wave resonator comprises a substrate 11 with electrodes 12A and 12B deposited on one of the major surfaces of substrate 11. The surface wave resonator may be a SAW employing a Rayleigh wave mode or a surface transverse wave (STW) resonator employing a shear-horizontal wave mode. The surface wave resonator is bonded to a circuit board 13 by an adhesive 14 that is applied uniformly over another major surface of substrate 11. The adhesive 14 may be a rigid adhesive such as an epoxy or a compliant adhesive such as RTV silicone. Electrodes 12A and 12B are coupled to circuit traces 15A and 15B with wire bonds 16A and 16B, respectively. The substrate 11 is a rectangular parallelepiped with a rectangular cross-section and rectangular plan-view.
Such prior art arrangements suffer from inherent disadvantages with respect to both normal, i.e. perpendicular to the major surfaces, and in-plane, i.e. in the plane of the major surfaces, acceleration sensitivities. Regarding the normal, or perpendicular, acceleration sensitivity, the prior art suffers from at least two distinct disadvantages. First, the simple rectangular shape of the device's cross-sectional and plan views does not permit optimal structural rigidity with respect to the flexural deformations of interest. Secondly, the application of adhesive over the full extent of a major surface, as well as the direct mounting of the resonator onto the circuit board provides little or no isolation from accelerations normal to the circuit board and flexural deformations of the circuit board. Concerning excessive in-plane acceleration sensitivity, the prior art arrangement has two somewhat similar disadvantages. The simple rectangular shape of the device's cross-sectional and plan views does not permit optimal structural rigidity to withstand the shear and anti-symmetric flexural deformations of interest. And, applying adhesive over the full extent of a major surface, as well as the direct mounting of the resonator onto the circuit board, provides little or no isolation from in-plane accelerations.
The two disadvantages of inadequate structural rigidity and lack of any degree of structural isolation contribute directly to excessively large acceleration sensitivity. This follows directly from the basic nature of acceleration sensitivity wherein the frequency shifts are directly proportional to the physical deformation of the crystal substrate. In turn, isolation from applied forces reduces the resulting deformation, as does an optimization of the structural rigidity of the resonator, both resulting in a lower acceleration sensitivity relative to a resonator lacking these features. Thus the inadequate structural rigidity and lack of structural isolation lead to deleterious effects on piezoelectric frequency stability, such as acceleration sensitivity and related effects including excessive aging due to mounting stress relief.
Thus, there has been a long-felt need to overcome the problems caused by the inadequate structural rigidity and excessive sensitivity to in-plane deformations in prior art surface wave piezoelectric resonators. A number of techniques have tried to resolve these long-standing difficulties with limited success, each obtaining some reduction in acceleration sensitivity at the price of increased size, weight, and so on. For example, one technique for eliminating or minimizing unwanted lack of rigidity and excessive sensitivity has been to introduce a simple, slab-like stiffener to the structure, but the use of such simple stiffeners has proven inadequate because of their enormous size relative to the crystal substrate; practical examples include 1 cm thick stiffeners applied to 0.35 mm thick resonators. Other alternative approaches to these long-standing problems, such as micro-machined isolation systems have also shown initial promise, but they come at the cost of additional size, sophisticated fabrication and possibly additional electronics to drive an active cancellation system.
Until now, there are no currently available lightweight, simple, and low-cost piezoelectric restraint mechanisms and packaging techniques that satisfactorily strengthen and stiffen the surface wave piezoelectric resonator against the unwanted lack of adequate structural rigidity and excessive sensitivity to in-plane deformations. The present invention provides an innovative restraint technique that combines a stiffening layer, adhesive and horizontal slotted indentations in a surface wave resonator that has solved the sensitivity problems that have retarded advances in piezoelectric equipment, without suffering from the disadvantages shortcomings and limitations of prior art devices.